Rhyming Quintains of Iambic Tetrameter
This poem has a pretty complicated form. We'll start with the (relatively) simple stuff. The poem consists of four stanzas with five lines each. These are called quintains. And in each quintain, the rhyme scheme is ABAAB. For example, take the first stanza:
Two roads diverged in a yellow wood, (A)
And sorry I could not travel both (B)
And be one traveler, long I stood (A)
And looked down one as far as I could (A)
To where it bent in the undergrowth; (B)
The rhythm of the poem is a bit trickier. It is basically iambic, which means that there is one unstressed syllable followed by a stressed syllable (da DUM). There are many variations in this poem, most of which are anapestic, which means that there are two unstressed syllables followed by a stressed syllable (da da DUM).
The most common use of iambs in poetry is in pentameter, which means that there are five "feet," or units of stressed and unstressed syllables, in the poem. But this poem is in iambic tetrameter, which means that there are only four feet (tetra = four). If you read the poem aloud, you should be able to hear four distinct beats per line. It will sound roughly like this: da DUM da DUM da DUM da DUM.
Let's look at the first line as an example. Stressed syllables are in bold and italic.
Two roads | diverged | in a yell|ow wood
Each of the four feet in this line is iambic except for the third, because both "in" and "a" are unstressed syllables, making it an anapest.
So this poem has a rhythm and rhyme scheme, but they depart a little from the norm, just like the speaker of this poem, who chooses his own path.